Microfacies Analysis Of The Jacksboro Limestone Member (Graham Formation, Virgilian, Pennsylvanian) In Jack County, Texas

The Jacksboro Limestone is a member of the Graham Formation of the Virgilian (Upper Pennsylvanian) and is exposed only in and around the city of Jacksboro in central Jack County, Texas. The purposes of this investigation were to study the vertical and lateral microfacies changes that occur throughout the Jacksboro Limestone in its outcrop belt and to discuss its relationship as the basal regressive cycle in the Virgilian part of the Pennsylvanian, in particular: (1) describe and classify the primary limestone lithofacies on the basis of carbonate rock textures, (2) interpret, if possible, the depositional environment and physical energy of the environment, based on the carbonate rock textures and allochemical constituents, and (3) describe the diagenetic properties and carbonate petrology of the Jacksboro Limestone. The primary sources of information for this investigation were collected in two large active limestone quarries, the Richards Pit and P&S Stone, as well as several other exposures located along a narrow northeast-southwest trending outcrop belt in the Jacksboro, Texas area.The basic criteria used to classify the limestone types were the classification systems of Folk (1959 and 1962) for thin sections and Dunham's (1962) for hand specimens. Based on binocular and petrographic examination of polished surfaces and thin sections, the carbonate beds of the Jacksboro Limestone are composed of several limestone textures. These textures are often mud supported with various proportions of fossil allochems but generally the carbonate mud content decreases and the proportions of fossils become more diverse from the base to the top of the studied sections. The limestone textures contain faunal and floral constituents including; fusulinids, foraminifers, brachiopods, gastropods, and crinoids that are commonly associated, but phylloid algae dominates the bulk of the skeletal constituents, except in a few beds.Field and laboratory analysis indicates that the Jacksboro Limestone formed on the sea floor as mound of loose accumulation of detrital skeletal grains and chemically and biologically precipitated carbonate mud in a warm, shallow, relatively low energy, marine environment. Insitu skeletal constituents and encrusting organisms existed but are not significant, thus the limestone mass is interpreted as a phylloid algal, bioclastic mound. Diagenesis of the carbonate sediments is widespread. Pressure solution, diagenetic minerals, and cementation are present but extensive. Recrystallization and dissolution are the most common alteration in the sediments; however, from a classification standpoint, this property does not alter the depositional texture of the limestone rocks.